This invention relates to a coating agent for a synthetic paper and a synthetic paper coated therewith.
Opaque or translucent films composed of a single layer of an inorganic fine powder-incorporated polyolefin or a laminate having at least one surface thereof such a powder-incorporated polyolefin layer, and especially such films in which the polyolefin layer has been oriented at least monoaxially are useful as synthetic papers.
Such synthetic papers, however, do not always have a satisfactory printability and an anti-staticity because the resin component used is a polyolefin. Accordingly, such synthetic papers are normally subjected to a suitable surface treatment. One of such surface treatments is to apply a coating agent on the surface of the synthetic paper.
A known example of such a coating agent is one containing a water-soluble antistatic polymer (e.g., Japanese Patent Laid-open Pub. No. 10624/75). This prior art coating agent is excellent in improving the adhesion of an ink and in overcoming the agglutination of dust due to static electrification or electrostatic charging. This coating agent is usually excessively water-soluble, however, and the resulting synthetic paper fails to give a satisfactory water-resistant adhesion of ink thereon and a satisfactory transfer of ink in off-set printing. Moreover there occur staining of printing plates and blurring of aqueous ink. Thus, such surface-treated synthetic paper has heretofore often lacked properties required for off-set printing paper. Such deficiencies are serious in view of the advantages of off-set printing.
In this connection, poor transfer of ink upon off-set printing occurs in multicolor continuous off-set printing and also in both-surface printing. The former case concerns the phenomenon of a nonuniform transfer of an ink in the second and succeeding color printings on a multicolor printing machine due to the influence of dampening water transferred onto the paper, which occurs as a result of strong retention of dampening water due to the strong affinity between the surface coating agent and water. The latter unevenness of ink transfer in the both-surface printing is due to a phenomenon of uneven transfer of an ink corresponding to the configuration of surface-printed images, which appears in the printing of the back surface of top surface-printed paper and occurs because the dampening water transferred onto the paper in the top surface printing undergoes migration to the back surface of the paper.
Off-set printing ingeniously utilizes the difference in affinity with printing plates between the water (dampening water) and an ink. The influence of the amount of the water is delicate and significant, as clearly shown by the fact that uneven transfer of ink takes place as a consequence of a small change in the amount of water.
The blurring of an aqueous ink occurs because the images stamped or written by a stamp ink, an aqueous ink pen, an aqueous ball-point pen, a recorder ink and the like take a long time before being completely dried. More specifically, the blurring is due to a phenomenon wherein the surface coating agent is dissolved by the water contained in the ink during the drying, and the water acts on the pigment in the ink to cause blurring of the ink, whereby the images written or stamped spread or become inflated until they are dried, sometimes to a degree such that the images become indistinguishable.
As one of the most successful synthetic papers, there has been a composite film having on the surface thereof a polyolefin film paper-like layer containing fine inorganic powder (e.g. clay and calcium carbonate). A paper of this character in which the surface layer has been stretched monoaxially and surface cracks exist at the sites of the inorganic fine powder particles has especially excellent properties in respect of off-set printability. It can be easily understood that the water-retention property on the surface of such a synthetic paper is affected by the water-retention or hydrophilic nature of the inorganic fine powder particles used. In this connection, clay has large water retention and calcium carbonate has small water retention. Therefore, it may be said that this synthetic paper having on the surface thereof a polyolefin film layer containing calcium carbonate has lower ability to absorb and retain the water content in an ink and dampening water transferred upon printing, and, as a consequence, uneven transfer of ink, blurring of an aqueous ink and the like are apt to take place more readily in comparison with a similar synthetic paper having a clay-containing layer.